SECTION VIII CONCENTRATION
BY EDWARD S.
Methods. — The chief principles which are employed or taken advantage of in concentration are: (1) That the rate of fall of fragmental solids through liquids and gases is proportional to specific gravity and size. (2) The interstitial effect already explained under grading but for particles all of the same specific gravity. To employ the interstitial effect in concentration it must be accompanied by cross or back washing. (3) Washing down or back effect with thin beds of grains on plane or curved surfaces. (4) Magnetic concentration. (5) Electrostatic concentration. (6) Those employed under the generic term "Flotation."
These principles with the exception of numbers (4) and (5) are arranged in the order of coarseness of particles or fragments. Principle number (1) is employed from 2-in. sizes down to sand sizes. Of late years however the tendency has been to employ principle (2) on the sand sizes and from J^-in. size down to about 80-mesh size. This change came about with the advent of the Wilfley table which employs interstitial action with cross washing. Jigs are the chief machine for employing principle (1). They work best with screened feeds. Below J^-in. size it was formerly the custom in Western metal-mine ore-dressing plants to prepare the feeds for the sizes finer than J^-in. with rising currents of water. To operate jigs properly with classified feeds requires experienced labor and the advent of the Wilfley table where the operations are all visible and require little training for its attendants has caused the almost entire displacement of classifier fed jigs. Indeed the tendency today in Western mill practice is to do away with jigging altogether, principally on the score of simplicity and to do away with the wear and tear which screens and jigs imply and the skilled labor required for properly operating jigs and other and more important but not so evident factors to be discussed at a later point. Whether or no jigs should be a part of any mill plan will depend upon what the testing leading up to the mill arrangements which this discloses. If the tests indicate that any shipping product can be made by jigging then the use of these machines is certainly indicated as a possibility. In many cases even if jigging does not yield a shipping product it may often be used to advantage as a roughing machine. Where the material being concentrated is crushed to half inch or finer the use of some type of rougher other than jigs may be preferable.
The chief machine and devices employing principle (3) are the round and canvas table. For sulphide ores and material which is adapted to saving by methods of notation the preference would be flotation rather than these devices if only one method is employed. The methods of flotation, while not the panacea for dressing problems that they were originally supposed, recover more in the size range to which principle (3) is applicable from about 80-mesh to the smallest which are visible to the eye. If desired flotation can precede machines employing this third principle or it can come after provided it is desired to recover an extra modicum which would not be yielded by flotation alone. Flotation like other methods is weak on the very finest material and while it is the practice to submit all the very finest material made to it provided it is of the kind which will "float" there are serious losses in the very finest sizes.
1 Consulting engineer, 407 Boston Building, Denver, Colo.